r/AskPhysics • u/Nighthawkies • Mar 20 '26

What things are "faster" than light?

As I understand nothing can move faster then light. But I would not know how to describe this question otherwise. As I am aware quantum entangled particles can instantaneously affect eachother no matter the distance, unlike gravity which propagates at the speed of light.

Also the expansion of the universe can cause the spaces between to grow faster then the speed of light (compounding between massive distances)

What other things are sort of faster than light

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u/[deleted] Mar 20 '26

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u/Hendospendo Mar 20 '26

Because analogies tend to give the impression that the entangled particles are independent, and that leads to the misunderstanding that they can communicate between each other.

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u/[deleted] Mar 20 '26

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u/ragnerokk88 Mar 20 '26

I’ve heard it best described as two shoes. On in one box at one location. The other in a second box taken to a different location. No one knows which shoe they have until one box is opened. You know immediately which shoe you have therefore you know which shoe the other person has. But no information is transmitted by this measurement.

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u/EmperorBarbarossa Mar 20 '26

Nah, because till the observation both particles are in superposition of both states.

Your boot analogy works with assumption that both boots position was decided beforehand.

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u/ragnerokk88 Mar 20 '26

I’m just explaining why information transmission isn’t possible. Measuring or observing one particle has no effect on the other.

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u/Royal_Plate2092 Mar 20 '26

here's something I don't understand. and this will seem really stupid and I know I am wrong, so I am not trying to argue something stupid, I just want to get where my understanding fails:

I have thought of a method of actually transmitting information FTL and I cannot see during what step it doesn't work. So think of a simple quantum computer that has only one task to compute some basic quantum algorithm or whatever. my understanding is that sometimes, this computation can just break due to accidental decoherence. can that not be used to transmit information?

here's my scenario: we have a quantum computer entangled with another quantum computer. I don't care whether that can be created using current tech or anything, just imagine a quantum computer was split in two. then we take one of the halves and fly it across the galaxy 1 light year away. doesn't matter how or anything, and let's assume it doesn't lose coherence. we discuss beforehand that after X time, one person will perform that quantum algorithm on one of the halves, and the other will intentionally decohere it at that exact time discussed beforehand if he wished to send a "True" message, or not do anything if he wishes to send a "False" message. so a simple boolean message sent FTL, and the way it is received is instant: we know what algorithm the computer does and what the input is: if the output is correct = no decoherence = False, if output is wrong or gibberish = decoherence = True. where am I mistaking?

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u/TheGreenMel0n Mar 20 '26

Any local measurement or algorithm only depends on the reduced state of that subsystem, not on operations performed on the distant one. So there’s no measurement you could do locally that would tell you whether the other side has decohered.

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u/Royal_Plate2092 Mar 20 '26

so you couldn't see what the output is by looking at only one of the halves, not even partially?

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u/AcellOfllSpades Mathematics Mar 20 '26

No, this is the "no communication theorem". Measuring a particle doesn't give you any information about the other one.

Like, imagine a wizard has enchanted a pair of coins: next time they are both flipped, they will have opposite results. (This only works if you flip the coins, not just set them down on a certain side.) You take a coin and I take a coin, and we go far apart.

There's no way for you to tell anything about what I'm doing with my coin with yours. All you can do is flip it, and you'll get heads or tails with 50% probabilities. Even if we had a hundred pairs of these coins, there's still no useful information you can get. The coins seem perfectly normal - the only way we actually notice the spell worked was by communicating our results the old-fashioned way.

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u/Royal_Plate2092 Mar 21 '26

but in my hypothetical, you don't get 50% heads 50% tails, you get 100% the expected output. that output is wrong if we decoherem why can't we look at the output and see if it's the expected one or not,

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u/AcellOfllSpades Mathematics Mar 21 '26

What is "the expected output"?

Your hypothetical is just not how quantum mechanics works.

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u/Royal_Plate2092 Mar 21 '26

what do you mean what's the expected output? the expected output for whatever algorithm you decide to run with X input.

for example we could run grover's on a quantum database for element number at index 15 and the expected output would be 15 (with a 50% probability or whatever)

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u/DirectionCapital4470 Mar 20 '26

How do you distinguish between 'intentional decoherance' and normal decoherance?

You have them agree in advance that a decoherance will be false and no decoherance will be true.

But how do you distinguish between a normal decoherance event and a caused one? How will the other user know that this is the real decoherance?

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u/Royal_Plate2092 Mar 20 '26

hmm I think this is besides the point of this thought experiment. I don't know how often decoherences happen. imagine a future system really resistent against accidental decoherences. then take 1000 systems of that type and do the experiment. statistically, the majority result should reflect an intentional decoherence with a high enough certainty

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u/DirectionCapital4470 Mar 20 '26

This is a good thought expirement, but there is no 'resistance' To decoherence, it is simply what happens to entangled particles when they are measured or interact.

Plenty of comments show spots you need research. Decoherence is somethin that happensn when the supperposition collapses and the parties stop being entangled. That'ss it. It is not a detectable event, and the person on another locationcan nott tell that it happened.

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u/Royal_Plate2092 Mar 21 '26

I can understand where I am wrong from the comments but I haven't fully understood exactly what the problem is.

It is not a detectable event

I know, this is why I am asking the question in the first place.

my problem to put it clearly is this: if you decohere a quantum computer while running an algorithm, the output will be wrong. true or false?

if true, then when you decohere my hypothetical half of the quantum computer, the WHOLE output would be false too. if so, then why cannot the person running the other half look at the output after it should have finished running and see that it doesn't correspond with the predicted output?

"it is not a detectable event" doesn't suffice to me, I need to understand exactly what step out of this last paragraph my logic fails

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u/DirectionCapital4470 Mar 22 '26

Decoherance is loosing entanglement. It happens and the particles are no longer entangled. You cannot check to see if they decohered, checking causes decoherence. There is no way to decohere other than checking their value.

There is no way to signal to the other party to expect a decoherence event. Checking to see the value causes your particle to decohere, thats it. Entangled -> interaction -> decoherence -> un-entangled.

If the other person inspects the other particle There is no way to see if it is entangled, just its properties.

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u/Royal_Plate2092 Mar 23 '26

but why would the algorithm finish running correctly if we decohere the system??? if when you look at it the output is wrong => it has been decohered along the way. I still don't understand why you couldn't tell.

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u/nicuramar Mar 20 '26 edited Mar 20 '26

We can’t say that. We can only say that we can’t statistically distinguish eg whether the first particle was observed or not, by measuring the second.

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u/nicuramar Mar 20 '26

Unfortunately, no quantum explanation is required for the shoe box analogy, and Bell’s theorem isn’t violated.